With high integration and large capacity of a Large Scale Integration (LSI), a circuit dimension required for a semiconductor element becomes increasingly narrowed. In the semiconductor element, during a production process, an original design pattern (that is, a mask or a reticle, hereinafter collectively referred to as a mask) in which a circuit pattern is formed is exposed and transferred onto a wafer by a reduction projection exposure apparatus called a stepper or a scanner.
It is necessary to improve a production yield of the expensive LSI in a production process. A shape defect of a mask pattern can be cited as a large factor that reduces a production yield of the semiconductor element.
A pattern having a line width of several tens of nanometers is required to be formed in the latest typical logic device. For example, the pattern constituting the LSI has become smaller, from the sub-micrometer order to the nanometer order as typified by a 1-gigabit class DRAM (Dynamic Random Access Memory). As a LCD (Liquid Crystal Display) grows in size with the progress of multimedia, a display with higher resolution capability is required. Specifically, the LCD is enlarged to the size of 500 mm×600 mm or more. On the other hand, the pattern of a TFT (Thin Film Transistor) provided on a liquid crystal substrate becomes smaller.
A shape defect of the mask pattern also becomes smaller according to the above-mentioned situation. Conventionally, fluctuations of various process conditions are absorbed by enhancing dimension accuracy of the mask. Therefore, it is necessary to detect the defect of an extremely small pattern in a mask inspection. Additionally, it is necessary to efficiently inspect a large-area LCD in as quickly as possible.
On the other hand, EUV lithography and Nanoimprint Lithography (NIL) attract attention as a technology for forming the fine pattern. In the EUV lithography, using extreme ultraviolet light as a light source, the pattern can be formed smaller than a conventional exposure apparatus in which ArF light is used. In the nanoimprint lithography, a fine pattern is formed in a resist by pressuring a mold (die) having a nanometer-scale fine structure to the resist on the wafer. In both the EUV lithography and nanoimprint lithography, the pattern formed in the mask and a template, which are of an original plate, is smaller than that of the conventional ArF lithography, and high inspection accuracy is therefore required in the inspection.
Therefore, in the inspection apparatus, a wavelength of illumination light is shortened in order to enhance a resolution capability. For example, deep ultraviolet light having wavelengths of 266 nm or less is used in a laser beam apparatus. However, the light emitted from the laser beam source becomes coherent light, and unfortunately a given interference fringe (speckle) is generated due to coherence.
FIG. 16 is a view illustrating a conventional illumination method.
FIG. 16 shows a lens array 1001 dividing a light beam from a light source (not illustrated) to generate a point light source group. The light becoming the point light source group is shaped into parallel light by a condenser lens 1002, and an object 1003 is illuminated with the parallel light. A position of the lens array 1001 corresponds to a Fourier plane of the object 1003, and a size of the lens array 1001 determines an illumination-side numerical aperture NA. At this point, although a resolution characteristic of an optical system is influenced by the illumination-side numerical aperture NA, generally the illumination-side numerical aperture NA is increased to the same level as a receiving-side numerical aperture NA in order to enhance resolution. Therefore, it is necessary that the point light source group generated by the lens array 1001 have a certain size.
In the optical system shown in FIG. 16, the light transmitted through the element lenses 1001a, 1001b, and 1001c constituting the lens array 1001 overlap one another on the object 1003 with different angles. For this reason, the interference fringe is generated on the object 1003 when the light incident to the element lenses 1001a, 1001b, and 1001c of the lens array 1001 have coherence. The number of point light sources generated by the lens array 1001 generally becomes a number between several hundred to several tens of thousands, and a wavefront of each piece of light is not necessarily an equiphase plane when the light transmitted through the element lenses 1001a, 1001b, and 1001c overlap one another on the object 1003. Therefore, the interference fringe has a random shape. The shape of the interference fringe fluctuates randomly due to an air fluctuation or a mechanical vibration. The randomly fluctuating interference fringe (speckle noise) loses a function of the illumination apparatus that needs to evenly illuminate the object 1003.
A technique has been attempted, in which the light from the light source is transmitted through a flyeye lens and transmitted through a rotating phase plate to eliminate the coherence of the coherent light, thereby reducing the speckle noise.
FIG. 17 is a view illustrating a conventional illumination method in which the rotating phase plate is used. In FIG. 17, the light emitted from the light source (not illustrated) and transmitted through a lens array 1004 is transmitted through a phase plate 1005, and shaped into a parallel light through a condenser lens 1006, and an object 1007 is illuminated with the parallel light. FIG. 18 is an enlarged sectional view of the region R in FIG. 17.
As illustrated in the enlargement shown in FIG. 18, the phase plate 1005 has an irregular shape in a section on the light incident side, and a random arrangement of the irregular shape of the phase plate 1005 provides a phase difference of 0 or π to each of the point light sources. As illustrated in FIG. 17, the interference fringe can be changed at high speed by a structure in which the phase plate 1005 is rotated by a rotating mechanism 1008. The speckle noise remaining in the captured optical image is reduced with increasing shape change of the interference fringe generated within an exposure time of an image sensor (not illustrated) that captures the image of the object 1007.
However, in the conventional illumination method, the speckle noise cannot be reduced beyond a limit defined by a rotating speed of the phase plate and the exposure time of the imaging element. In order to reduce the speckle noise, it is necessary to enhance the rotating speed or lengthen the exposure time of the imaging element. There is a limit to the enhancement of the rotating speed, and the lengthened exposure time leads to a degradation of throughput. Therefore, both these methods are hardly used.
The conventional illumination apparatus has the insufficient resolution capability required in association with the smaller pattern of the semiconductor element, and there is a strong demand to develop the illumination apparatus that can solve the problem and the inspection apparatus provided with the illumination apparatus. An object of the present invention is to provide an illumination apparatus that can reduce the speckle noise more than before. Another object of the present invention is to provide an inspection apparatus that allows detection of fine defect, required in association with the smaller semiconductor element to be performed efficiently in the shortest possible time.
Other challenges and advantages of the present invention are apparent from the following description.